The present invention relates to a magnetic head assembly and, in particular, to an assembly for reading and writing to a plurality of tracks on a magnetic medium.
The continuing need in the data processing and computer fields for larger memories creates an increasing demand on non-volatile mass storage devices such as magnetic tape used for backing up data. A mass storage unit offering higher capacity should also offer "backwards compatibility." For a magnetic tape, backwards compatibility implies the ability to at least read extant tapes recorded at the lower density with the new high capacity tape drive. Backwards compatibility is important because the number of extant tapes can be great. Thus, converting data on numerous extant tapes into higher density tapes compatible with a new drive would be prohibitively expensive. Also a user may continue to create lower density tapes by operating separate machines with the higher and lower density tape drives. The operator may wish to transfer data recently written by the lower density tape drive to the system having the improved performance. The latter is referred to as "backwards read compatibility."
Also desirable is "backwards write compatibility," the ability of the higher density tape drive to write in not only the higher density mode, but in the earlier low density mode. Therefore, the high density tape drive could be used to transfer data to a machine having a low density tape drive.
Both types of compatibility, especially backwards write compatibility, can be difficult to achieve with tapes that are simultaneously written with multiple parallel channels across the width of the tapes. Often the increased data density is achieved by increasing the number of channels on the tape. Consequently, there potentially will be a physical incompatibility between earlier tape and the improved, high density magnetic head.
In a conventional 18 track tape drive, the write heads write 0.54 mm wide tracks at a pitch of 0.63 mm. The read heads are aligned with the write heads and are 0.41 mm wide. The latter are made narrower to allow for manufacturing tolerances, imperfect tracking of tape, and differences between machines, to assure that the read heads will always be over good sections of track, even when the tapes are written on different drives.
A simple but problematic approach towards compatibility would be to provide twice the number of read and write elements, but operated in pairs. Such a design must deal with both electromagnetic and mechanical limitations. One of the mechanical limitations in fabricating a high density magnetic head is the conventional step of closely cutting narrow slots in ferrite material to define the border between adjacent channels. For example, practical cuts may be kept no less than 0.09 mm wide. Reducing the pitch and width of the cuts to increase data density can be technologically difficult and can raise fabrication costs unacceptably.
Electromagnetically, if an 18 track tape were loaded on a 36 track machine and the outputs of selected pairs of read heads were combined, tracks could be successfully read. Since output is proportional to head width, all other things being equal, the output would be smaller than with the 18 track head. The width of the conventional read element is 0.41 mm, while the combined width of a pair of elements from a 36 track head would be about 0.23 mm. However, with proper care, cable shielding and preamplifier design, the signal to noise ratio would be adequate to recover the data. Thus a pair of high density read elements could be connected together to read an older tape whose wider channels would each extend over a pair of read elements.
High density write elements could also be driven as a pair to simulate a relatively wide track. However, recording in this fashion will cover only part of the traditional channel width. The new pair of write elements will not record in the space between them, which would be the center of the simulated wide channel. Accordingly, the signal to noise ratio (hereinafter signal to interference ratio) would be severely degraded. This ratio could be as low as 11.02 dB, if a double density drive (18 to 36 tracks) wrote a pair of tracks at a combined width of 0.41 mm leaving a 0.09 mm interfering strip. Such interference would prevent reliable operation. Filtering this interference would be impossible since the interference would have the same spectrum as the desired signal. Thus, a user would be obliged to make certain the tapes were erased before recording if they could possibly contain full-width, 18 track data.
In the foregoing example, the intertrack gap was unchanged from what it was in the earlier machine. If, however, all dimensions could be reduced equally, the intertrack gap could also be cut in half, for example, to 0.045 mm. The track width simulated by the paired write heads would remain at 0.41 mm. With these assumptions the signal to interference ratio would be 18.18 dB, a significant improvement. Data integrity, however, would still not be sufficient. Moreover, practical manufacturing methods may not allow reducing the intertrack gap because it may require cutting an unacceptably narrow slot of 0.045 mm.
Accordingly, there is a need for an improved high density magnetic head that can provide backwards read and write compatibility with earlier machines having fewer channels on the magnetic media.